Tetrafluoroethane isomerization

ABSTRACT

A method for the preparation of 1,1,1,2-tetrafluoroethane whereby 1,1,2,2-tetrafluoroethane is contacted with the fluorination catalyst at elevated temperature to effect isomerization.

This invention relates to a chemical process and more particularly to aprocess for the manufacture of 1,1,1,2-tetrafluoroethane.

Several methods have been proposed for the manufacture of1,1,1,2-tetrafluoroethane which is a useful refrigerant, aerosolpropellant, blowing agent and solvent. Thus, our United Kingdom Pat. No.1578933 describes a process for making tetrafluoroethanes byhydrogenating dichlorotetrafluoroethanes at elevated temperatures, forexample temperatures in the range 200°-450° C. Also, our United KingdomPat. No. 2004539 describes the manufacture of 1,1,1,2-tetrafluoroethaneby reacting trifluoroethylene in the vapour phase with hydrogen fluoridein the presence of chromium oxide, suitable reaction temperatures beingin the range 200°-500° C. Both of these processes are expensive tooperate.

It has now been found that 1,1,1,2-tetrafluoroethane can be prepared inhigh yield and high selectivity by the isomerisation of1,1,2,2-tetrafluoroethane under the conditions hereinafter described.

Thus, according to the invention, there is provided a method for thepreparation of 1,1,1,2-tetrafluoroethane which comprises contacting1,1,2,2-tetrafluoroethane with a fluorination catalyst at an elevatedtemperature whereby to effect isomerisation.

Fluorination catalysts useful in the method of the invention have beenfully described in the prior art and include aluminium fluoride, sodiumfluoride, gammaalumina and, especially, chromia. Unmodified chromia maybe used but it is advantageous to use a pre-fluorinated chromia that hasbeen used in other fluorination reactions. After use, the catalyst maybe regenerated by treatment with air at an elevated temperature, forexample 400°-420° C.

The isomerisation temperature should be sufficiently high to effectconversion of the 1,1,2,2,-tetrafluoroethane but not so high thatexcessive amounts of by-products, such as pentafluoroethane, areobtained. In general, suitable temperatures are found in the range fromabout 300° to about 900° C. Lower temperatures can generally be employedwhen using chromia as the fluorination catalyst, for exampletemperatures from about 300° to about 550° C., more especially from 350°to 500° C. and preferably from 400° to 450° C. At these temperatures,the 1,1,2,2-tetrafluoroethane, optionally in conjunction with an inertdiluent such as nitrogen, is suitably contacted with the catalyst fortimes of between about 1 and about 50 seconds.

The 1,1,2,2-tetrafluoroethane use as starting material in the method ofthe invention may be obtained by known methods, for example by themethod described in United Kingdom Pat. No. 1578933 or by thehydrogenation of tetrafluoroethylene. The latter reaction may beconveniently effected at normal or elevated temperatures, for example upto 250° C., in the presence of a hydrogenation catalyst, for examplepalladium on alumina.

The invention is illustrated but not limited by the following Example.

EXAMPLE Hydrogenation of Tetrafluoroethylene

Hydrogen at 1000 ml/min and tetrafluoroethylene at 100 ml/min werepassed over 120 g 5% palladium/alumina at ambient temperature in a glassreactor, the off gases being collected in two traps cooled intrichloroethylene and Drikold. There was an immediate exotherm and thetemperature rose to 250° C. After 210 min, the hydrogen andtetrafluoroethylene flows were turned off and the reactor and linespurged with nitrogen. The material condensed in the traps was weighedand analysed. The product was 84.4 g (94% yield) of1,1,2,2-tetrafluoroethane (purity 98.5%).

Isomerisation of 1,1,2,2,-Tetrafluoroethane

1,1,2,2-Tetrafluoroethane and diluent nitrogen were passed through aHastelloy reactor tube packed with 80 ml chromia, the reactor beingheated in a furnace. After the reactor, the gases were passed through anempty tube to effect cooling and then through a scrubber containing 25%potassium hydroxide solution to remove hydrogen fluoride. The productswere finally condensed in a trap cooled in trichloroethylene andDrikold. Gas samples were taken after the scrubber for analysis by gaschromatography.

Details of a number of runs using pre-fluorinated chromia, which hadbeen dried at 400° C. for 4 hours and then "activated" by passage of airat 200-300 ml/min at 400° C. overnight, are given below in Table 1. Inthe Table:

A134=1,1,2,2-tetrafluoroethane

A134a=1,1,1,2-tetrafluoroethane

3FE=trifluoroethylene

A125=pentafluoroethane

A flow rate of 50 ml/min A134 and 100 ml/min nitrogen corresponds to acatalyst contact time of approximately 12 sec.

Table 2 gives details of runs using the same catalyst after regenerationwith 400 ml/min air at 400° C. for 16 hours.

                  TABLE 1                                                         ______________________________________                                               Tem-                                                                   Catalyst                                                                             pera-  A134     Nitrogen                                                                             Concentration in Off Gas                        Volume ture   Flowrate Flowrate                                                                             mole %                                          ml     °C.                                                                           ml/min   ml/min A134 A134a 3FE  A125                            ______________________________________                                        80     288    50       100    93.5  1.0  0.1  0.2                             80     325    50       100    91.4  3.0  0.1  0.4                             80     420    50       100    69.9 21.5  0.2  0.7                             80     460    50       100    59.4 34.1  0.3  1.8                             80     497    50       100    52.6 34.6  0.2  8.5                             80     550    50       100    48.1 28.1  0.3  27.9                            80     510    50       100    56.0 26.0  0.6  12.7                            80     465    50       100    73.4 21.0  1.0  2.7                             80     410    50       100    88.5  9.1  0.6  0.4                             80     470    50       100    74.9 19.4  0.2  3.8                             ______________________________________                                    

                  TABLE 2                                                         ______________________________________                                        80     420     50     100   29.1   4.2  --   60.2                             80     343     50     100   66.9  25.7  0.4  3.7                              80     328     50     100   75.5  21.1  --   1.1                              80     375     50     100   43.6  48.8  0.1  5.4                              80     388     50     200   52.3  43.0  0.2  2.8                              80     400     50     300   46.3  48.1  0.1  3.9                              80     412     50     400   49.8  44.3  0.1  4.4                              80     410     50     200   40.3  50.2  0.9  7.1                              80     430     50     300   47.1  43.6  1.6  6.2                              80     440     50     400   53.1  36.8  2.2  6.5                              80     445     50     100   28.4  52.9  0.5  16.6                             80     450     50     150   41.4  45.1  0.7  11.1                             80     455     50     100   40.6  42.5  0.6  14.7                             80     455     50     100   47.9  42.2  0.8  7.9                              ______________________________________                                    

I claim:
 1. A method for the preparation of 1,1,1,2-tetrafluoroethanewhich comprises contacting 1,1,2,2-tetrafluoroethane with a fluorinationcatalyst at an elevated temperature whereby to effect isomerisation. 2.A method according to claim 1 wherein the catalyst comprises chromia. 3.A method according to claim 2 wherein the catalyst comprisespre-fluorinated chromia.
 4. A method according to any preceding claimwherein the temperature is in the range from about 300° to about 900° C.5. A method according to claim 4 wherein the temperature is in the rangefrom about 300° to about 550° C.
 6. A method according to claim 5wherein the temperature is in the range from 350° to 500° C.
 7. A methodaccording to claim 6 wherein the temperature is in the range from 400°to 450° C.